Anyone in Plumbing/Home building/Construction? I need some help!

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I just purchased a NEW two story townhome 10 months ago. It is a building of 6 units and we are on the end. Starting around Christmas, our home began to smell bad after it rains. The smell is of like rotten eggs. It's disgusting. The rain will stop and a day later, the smell will be gone. It comes from our kitchen pantry (downstairs). There is no sign of water intrusion. The walls are not wet and the builder came out with the infrared camera to check for moisture behind the walls and nothing showed up. My wife and I thought it was food, but I smelled the wall and it's coming from inside the wall! I called our builder and they cut big holes in the wall. The a/c drain line that runs under the foundation to the a/c unit at the back of the house was not foamed in properly allowing some air/gases inside so they foamed it in. I thought this solved the problem, but it's raining right now and my house STINKS again!! It smells like a dead f-ing animal. It's ONLY when it rains. We're on City, not septic. We do not have natural gas, only electricity. The builder said it could be a dried up P-trap?? but my wife's father said those take a year or more to dry out? My neighbors have no smell. This is in Florida, so no basement or fireplace either.
Any ideas wtf this is? [img]/images/graemlins/frown.gif[/img]